Delivery factors and neonatal thyroid hormone levels: a systematic review

Postnatal thyroid hormone changes enable the neonate to be adapted for postnatal life. Several factors can affect this adaption. In this review, we summarized the studies that reported the association among the delivery factors and neonatal thyroid hormones and thyroid stimulating hormone. A comprehensive literature search was performed in PubMed, Web of Science, and Scopus up to March 2020, to identify the studies investigating the relationship between delivery factors, especially the mode of delivery and labor pain, and neonatal thyroid hormones and TSH. Finally, in this review study, of 157 articles obtained in the initial search, 25 eligible studies were reviewed. Various maternal, fetal/neonatal, and obstetric factors affected neonatal TSH and thyroid hormones. Among various influencing factors, mode of delivery, labor pains, and duration of labor have a strong relationship with neonatal thyroid hormones and need to be considered for interpretation of neonatal thyroid status. The majority of the studies revealed that vaginal deliveries (instrumental or natural) lead to higher levels of cord TSH compared to elective cesarean section. This can be explained by the increased secretion of catecholamine during labor. It has been suggested that blood TSH and thyroid hormone levels are affected by perinatal stress events such as maternal anxiety, labor pains, fetal distress, and other stimulants of the catecholamine response. These changes may act as the trigger to increase thyroid hormone levels for adapting of neonates in the first hours after birth. This assumption needs to be re-evaluated by performing comprehensive and well-designed studies.